Wireless communication systems, such as the 3rd Generation (3G) of mobile telephone standards and technology, are well known. An example of such 3G standards and technology is the Universal Mobile Telecommunications System (UMTS™), developed by the 3rd Generation Partnership Project (3GPP™) (www.3gpp.org). The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Such macro cells utilise high power base stations (Node Bs in 3GPP™ parlance) in order to communicate with wireless communication units within a relatively large geographical coverage area. Typically, terminal devices (sometimes referred to as wireless communication units, or User Equipment (UEs) as they are often referred to in 3G parlance, communicate with a Core Network (CN) of the 3G communication system via a Radio Network Subsystem (RNS). A wireless cellular communication system typically comprises a plurality of radio network subsystems, each radio network subsystem comprising one or more (coverage) cells to which UEs may ‘attach’, and thereby connect to the network. Each macro-cellular RNS further comprises a controller, in a form of a Radio Network Controller (RNC), operably coupled to the one or more Node Bs, via a so-called Iub interface.
The 3GPP has subsequently defined a further generation of mobile telephone standards, known as Long Term Evolution (LTE), which is commonly referred to as 4G. In LTE, a base station is known as an Evolved Node B (abbreviated to eNodeB or eNB). An eNB connects directly to the LTE core network, which is known as the Evolved Packet Core (EPC), there being no equivalent in LTE of the 3G RNC.
Lower power (and therefore smaller coverage area) cells are currently referred to as ‘small’ cells, with the term femto cells or pico cells typically reserved to refer to a residential small cell. Hereafter, the term small cells will be used to encompass femto cells, pico-cells or similar. Small cells are effectively communication coverage areas supported by low power base stations (otherwise referred to as Access Points (APs) with the term Home Node Bs (HNBs) identifying femto cell access points). These small cells are intended to augment the wide area macro network and support communications to UEs in a restricted, for example indoor, environment. An additional benefit of small cells is that they offload of traffic from the macro network to small cells, thereby freeing up valuable macro network resources.
Typical applications for such femto APs/HNBs include, by way of example, residential and commercial (e.g. office) locations, communication ‘hotspots’, etc., whereby APs/HNBs can be connected to a core network via, for example, the Internet using a broadband connection or the like. In this manner, small cells can be provided in a simple, scalable deployment in specific in-building locations where, for example, UEs may come into close proximity to a small cell base station. Small cell base stations are intended to enhance the coverage of a UMTS™ Radio Access Network (RAN) within residential and/or private commercial environments, and it is planned that the number of small cell base stations in a macro cell may number thousands. Such small cell base stations may include a Network Listen (NWL) function, whereby the basestation receives and decodes the downlink transmissions from other basestations for the purpose of identifying neighbour cells and their characteristics, though this technique has the limitation that the signal strength or signal quality of a neighbour cell is determined at the location of the small cell base station rather than at locations at which UEs may be located.
In order to support mobility of a UE between cells (for example, handover, cell reselection and RRC redirection), the UE typically has to be able to perform measurements on the signals from neighbouring cells, and generally the network has to provide the UE with information about these cells in the form of neighbour cell lists. It is known in 3GPP™ that Small Cells are often deployed with minimum RF planning and in some cases are installed by consumers within their homes in an adhoc fashion, which can make it difficult for the small cell base station to be given or to obtain complete and accurate information about neighbouring cells. Neighbour cell lists may be based upon: (i) Manual operator provisioning; (ii) NWL mode measurements and (iii) UE measurements. In addition, without correct neighbour cell lists the UE may fail to reselect back to the macro network, may fail to handout the call (resulting in a call drop) or may be redirected to the wrong cell (resulting in call establishment failure). For reliable handover in particular, it is advantageous if a UE can quickly measure the cell or cells to which it is most likely to be able to handover successfully. The more cells a UE has to measure, the longer it takes, so it is advantageous for the AP/HNB only to ask a UE to measure a small number of cells, but the list must be optimised to include those that are most likely to yield handover success.
In general, UMTS UEs support measurement of (a) intra-frequency cells, which are UMTS cells on the same frequency as the serving cell (b) inter-frequency cells, which are UMTS cells on frequencies other than that of the serving cell (c) inter-RAT cells, which are cells that use radio access technologies other than UMTS, such as GSM or LTE. UMTS UEs support the ability to detect and supply measurement results for neighbouring intra-frequency cells about which they have been given no information in advance: such cells are known as ‘detected set cells’. UMTS UEs conforming to 3GPP Release 9 or earlier cannot report UMTS detected set inter-frequency cells. UMTS UEs conforming to 3GPP Release 10 may be able to report detected set inter-frequency cells. UMTS UEs cannot report detected set inter-RAT cells.
If a small cell base station does not have complete information about inter-frequency or inter-RAT neighbour cells, it can in principle perform ‘blind neighbour cell detection’ by giving a UE information for a cell or cells that might or might not exist: the UE will only report measurement results for cells that do actually exist. In this way, the small cell base station can in effect ask a UE to scan through lists of available operating frequencies to find neighbouring cells. Typically the small cell base station has to command a UE to enter ‘compressed mode’ in order to measure inter-frequency and inter-RAT cells. Since ‘compressed mode’ uses more radio resources, UEs are typically only asked to measure inter-frequency and inter-RAT cells when they are near the edge of a cell, where handover to a neighbour cell is likely to be required. In such circumstances, it is important for the UE to be able to measure the best neighbour cell for handover quickly, and therefore it would be disadvantageous to ask the UE to perform measurements for any other purpose (including, but not limited to, ‘blind neighbour cell detection’) at the same time. Typically, the opportunities for asking UEs to perform measurements other than those specifically needed to support handover are both too few and too limited in duration to support neighbour cell list optimisation for Small Cells.